Vehicle crashes caused by drunk drivers have been a concern for many years. It has been proposed to equip vehicles with blood alcohol concentration (BAC) sensors to estimate the BAC of a person attempting to operate a vehicle based on the concentration of ethanol gas in the breath of the person. One conventional method to measure BAC of a person requires the person to fully exhale into a mouthpiece coupled to an apparatus that uses a fuel cell sensor to measure the ethanol vapor concentration in the exhaled breath emerging near the end of the exhalation. However, some people find this unpleasant. Other conventional methods and apparatus for measuring BAC do not require a person to exhale into a mouthpiece, but rely generally on a valve to route a second source of air without exhaled breath or without ethanol gas to serve as a baseline for determining ethanol gas concentration. Other methods rely on complicated signal analysis techniques that compare waveforms from an ethanol gas detector and a carbon dioxide gas detector. Moreover, some methods require a period time on the order of a minute to output an estimate of BAC and are sensitive to ambient concentrations of ethanol vapor such as from an intoxicated passenger or spilled ethanol fuel.